Floral initiation is controlled by several factors including photoperiod, cold treatment, hormones, and nutrients (Coen, Plant Mol. Biol. 42:241-279, 1991; Gasser, Annu. Rev. Plant Physiol. Plant Mol. Biol. 42:621-649, 1991). Physiological studies have demonstrated that vegetative tissues are the site for signal perception and for generation of chemicals that cause the transition from vegetative growth to flowering (Lang, in: Encyclopedia of Plant Physiology, vol. 15, Berlin, ed., Springer-Verlag, pp. 1371-1536, 1965; Zeevaartm, in: Light and the Flowering Process, Vince-Prue et al., eds., Orlando Academic Press, pp. 137-142, 1984). Genetic analysis revealed that there are several types of mutants that alter flowering time. In Arabidopsis thaliana, there are at least two mutant groups based on their response to photoperiod and vernalization (Martinez-Zapater et al., in: Arabidopsis, Meyerowitz and Somerville, eds., Plainview, N.Y., Cold Spring Harbor Laboratory, pp. 403-433, 1994). These phenotypes suggest that there are multiple pathways that lead to flowering.
Study on mutants that interfere with normal flower development has provided some information on controlling the mechanisms of the development. This has led to the knowledge that there are at least two genes needed for induction of flower development: LEAFY (LFY) and APETALAI (API) genes in Arabidopsis (Weigel, Annu. Rev. Genet. 29:19-39, 1995), and FLORICAULA (FLO) and SQUAMOSA (SQUA) genes in Antirrhinum majus (Bradley et al., Cell 72:85-95, 1993). Cloning and analysis of these genes revealed that the LFY and FLO genes are homologs and encode proteins that each contain a proline-rich region at the N-terminus and a highly acidic central region, which are features of certain types of transcription factors that contain a conserved MADS-box sequence (Huijser et al., EMBO J. 11:1239-1249, 1992; Mandel et al., Nature 360:273-277, 1992). MADS box-containing genes were isolated from several plant species and are known to play important roles in plant development, especially flower development. Arabidopsis homeotic genes--AGAMOUS (AG), PISTILATA (PI), and APETALA3 (AP3)--are members of the MADS box gene family (Yanofsky et al., Nature 346:35-39, 1990; Goto and Meyerowitz, Genes Devel. 8:1548-1560, 1994; Jack et al., Cell 68:683-697, 1992). Similar homeotic genes from A. majus--PLENA (PLE), GLOBOSA (GLO), and DEFICIENS A (DEFA)--are also MADS box genes (Bradley et al., Cell 72:85-95, 1993; Trobner et al., EMBO J. 11:4693-4704, 1992; Sommer et al., EMBO J. 9:605-613, 1990). Characterization of these gene products showed that the conserved MADS box domain is for sequence-specific DNA binding, dimerization, and attraction of secondary factors (Pellegrini et al., Nature 376:490-498, 1995). The DNA sequence with which the MADS box domains interact is the consensus finding site, CCA/T.sub.6 GG (Pollock and Treisman, Genes Dev. 5:2327-2341, 1991; Huang et al., Nucl. Acids Res. 21:4769-4776 1993). In addition to the MADS-box domain, the plant MADS box proteins include the K-box domain, a second conserved region carrying 65-70 amino acid residues. The K-box domain was named due to the structural resemblance to the coiled coil domain of keratin (Ma et al., Genes Dev. 5:484-495, 1991) and has been suggested to be related to protein-protein interactions (Pnueli et al., Plant J. 1:255-266, 1991). Similar MADS-box genes have also been studied in other plants including tomato, rape, tobacco, petunia, maize, and rice (Thei.beta.en and Saedler, Curr. Opin. Genet. Dev. 5:628-639, 1995). A number of plant MADS box genes that deviate from the functions of the typical meristem identity and organ identity genes have been identified. These genes are involved in the control of ovule development (Angenent et al., Plant Cell 7:1569-1582, 1995), vegetative growth (Mandel et al., Plant Mol. Biol. 25:319-321, 1994), root development (Rounseley et al., Plant Cell 7:1259-1269, 1995), embryogenesis (Heck et al., Plant Cell 7:1271-1282, 1995), or symbiotic induction (Heard and Dunn, Proc. Natl. Acad. Sci. USA 5273-5277, 1995).
There are a large number of MADS box genes in each plant species. In maize, at least 50 different MADS box genes consist of a multigene family and these genes are dispersed throughout the plant genome (Mena et al., Plant J. 8:845-854, 1995; Fischer et al., Proc. Natl. Acad. Sci. USA 92:5331-5335, 1995). The MADS box multigene family can be divided into several subfamilies according to their primary sequences, expression patterns, and functions (Thei.beta.en and Saedler, Curr. Opin. Genet. Dev. 5:628-639, 1995).
The timing of the transition from vegetative growth to flowering is one of the most important steps in plant development This step determines the quality and quantity of most crop species by affecting the balance between vegetative and reproductive growth. It would therefore be highly desirable to have means to affect the timing of this transition. The present invention meets this and other needs.